1. Field of the Invention
The present invention relates to a process for manufacturing wafers of semiconductor material by layer transfer. In the following description particular reference is made to the fabrication of silicon-on-insulator (SOI) wafers, without, however, the invention being limited thereto.
2. Description of the Related Art
As is known, SOI wafers are formed by two silicon layers, one of which is thinner than the other, separated by a silicon oxide layer. Lately, SOI wafers have received considerable attention because integrated circuits having substrates formed from these wafers have considerable advantages as compared to the same circuits formed on traditional substrates formed only by monocrystalline silicon.
A typical process for manufacturing SOI wafers, described in the article “Silicon-on-Insulator Wafer Bonding-Wafer Thinning Technological Evaluations” by J. Hausman, G. A. Spierings, U. K. P. Bierman and J. A. Pals, Japanese Journal of applied Physics, Vol. 28, No. 8, August 1989, pp. 1426-1443, is based upon the bonding of two wafers of monocrystalline silicon (wafer-bonding process). The wafers obtained with the traditional method of wafer bonding have excellent electrical characteristics but decidedly high costs. Another problem lies in the fact that the grinding operations necessary for thinning one of the two silicon layers are costly and delicate on account of the brittleness of the material. For the same reason, the yield of the process is not very high.
A process called “smart cut™” has then been proposed (see, for example, “Hydrogen-induced silicon surface layer cleaverage” Xian Lu, Nathan W. Cheung, Michael D. Strathman, Paul K. Chu, Brian Doyle, Appl. Phys. Lett., Vol. 71, No. 13, Sep. 29, 1997, pp. 1804-1806), which is based upon the bonding of two wafers, whereof a first one has undergone a hydrogen implant, which, during an appropriate annealing step, causes the formation of microcavities filled with hydrogen molecules. By subjecting the composite wafer to an annealing step at an appropriate temperature, the microcavities grow on a horizontal plane parallel to the surface of the composite wafer and cause the separation of the first wafer (which has undergone implantation) into two parts, one of which is bonded to the second wafer. Thus, a SOI wafer is obtained, having a monocrystalline silicon layer, the thickness whereof depends upon the implantation depth. The SOI wafer thus obtained is then subjected to lapping, to obtain the desired smoothness.
The above process, however, proves very costly on account of the operations of hydrogen implantation and lapping.